1. Field of the Invention
The invention relates to a housing part for a measurement device having a microwave-permeable glass or ceramic window.
2. Description of the Related Art
Measurement devices are often installed in explosion-hazard environments or are mounted to plant components, such as vessels or pipework, with high pressure and temperature inside. The housing of such measurement devices may be equipped with a glass or ceramic window for observation purposes or as a lead-through for optical or microwave signals out of the housing into the environment or plant component and/or vice versa. In order to withstand the pressure from inside the plant component or resulting from an explosion within the housing, the window is hermetically sealed to the part of the housing in which it is disposed. Depending on requirements, the housing may be hermetically sealed, pressure-resistant or explosion-proof.
U.S. Pat. No. 4,961,628 discloses a disk-type window assembly for observing the interior of a gas-tight and pressure-tight closed chamber, where the window assembly comprises a cylindrical frame in the form of an outer ring made of metal and a window in the form of a transparent insert arranged within the frame. The frame is preferably made of stainless steel, and the insert is made of glass, such as borosilicate glass, which is fused into the frame. Having a higher coefficient of thermal expansion than that of the glass insert, the metal outer ring will exert a radial compression force on the glass after the window assembly is cooled down from the fusing temperature. This creates radial compression stresses that preload the glass window and provide it with the ability to withstand various loading conditions during operation, such as bending due to pressure on one side or a sudden change of temperature received on the glass surface. The glass window only cracks when the precompression stresses are completely annulled by the stresses from operating conditions.
Such glass window assemblies are typically mounted and sealed (e.g., by screwing or clamping) to the wall of a container, a vessel or a housing to form a fully sealed enclosure. An example is the case of an explosion proof enclosure used in electrical applications where the glass window has to be designed to withstand the internal explosion pressure.
US 2009/0114012 A1, for example, discloses a radiometric filling level indicator having an explosion-proof housing and compression glass lead-through with the glass fused into a frame. The frame has an outer thread to screw the compression glass lead-through to the housing.
U.S. Pat. No. 6,639,745 B1 discloses an observation window of a hyperbaric oxygen chamber, where a holding frame for the window is joined to a wall of the chamber via solder welding. The frame has an annular holding portion defining a central observation hole into which the window is closely fitted. The window is secured from falling out by an O-ring, which further prevents gas from traveling through the joint between the window and the frame. The frame has an annular space formed between its outer annular connecting portion and an outward section of the inner annular holding portion of the frame for reducing effects of deformation of the outer annular connecting portion on the inner holding portion that may be caused by high temperature of the solder welding.
DE 199 10 270 A1 discloses a window assembly to be used in a radar level gauge, where instead of fusing a glass or ceramic into a metal frame, a metal jacket is fused or sintered to the rim of a microwave-permeable window.
U.S. Pat. No. 6,359,742 B1 discloses a sight glass assembly adapted for welding to a vessel wall. The sight glass assembly comprises a transparent window disc, an annular frame that is fused to and encloses the window disc, and a flange. The flange is thinner than the frame and extends radially from one edge of the frame to form a flush surface coextensive with a surface of the window disc and the frame. The flange has an annular groove surrounding the frame and window disc. As the perimeter of the flange is welded into an opening of the vessel wall, thermal expansion might create stress throughout the assembly. However, the groove will absorb the stresses without transmitting to the glass viewing portion of the assembly and thus protects the integrity of the entire unit.
US 2002/0067229 A1 and U.S. Pat. No. 5,770,990 each disclose a microwave window assembly for spatial separation and microwave-transmitting connection between an external microwave conductor and an internal microwave conductor or horn antenna of a radar level gauge. The external microwave conductor is outside a container holding a medium, the level of which is to be measured. The window assembly comprises a microwave-permeable window and a metal frame into which the window is fused or glued. The window assembly, or more precisely its frame, is mounted or bolted directly between flanges of the microwave conductors or, as shown in U.S. Pat. No. 5,770,990, between the flange of the external microwave conductor and the wall of the container. U.S. Pat. No. 5,770,990 also discloses the possibility of sealing or sintering the window directly into the wall of the container. In order to adjust the impedance of the microwave window to the impedance of the external microwave conductor or the internal microwave conductor, cone-shaped matching elements made of dielectric material may be provided on both sides of the microwave window.
US 2002/0053238 A1 discloses a housing part for a microwave level measuring device which may be mounted on a wall of a container at a measurement hole through it. The housing part comprises a mounting flange into which a metal waveguide jacket is inserted and welded. The metal jacket has a flat center portion of reduced wall thickness with a central hole into which a microwave-permeable glass or ceramic window is fused. The window forms flush surfaces coextensive with the upper and lower surfaces of the jacket's center portion. A hollow waveguide is mounted in the upper side of the jacket. For better matching of the microwaves with a horn antenna, a tapered dielectric matching element is attached to the lower side of the jacket with a holding ring that has an external thread and is screwed into an internal thread section of the jacket.
EP 1 009 650 A1 discloses a manhole cover plate with a central orifice into which a sight glass is fused. A concentric groove on the upper side or lower side of the plate around the glass prevents stress on it.
U.S. Pat. No. 2,744,592 discloses a flat circular waveguide window comprising a glass center and a surrounding metal frame to which the glass is fused. The metal frame is composed of an annular outer frame portion and an inner frame portion. The outer and an inner frame portions are connected by a zone of reduced thickness that reduces the possibility of transmission of cooling stresses from the outer frame portion to the glass center.